Stem cells are defined as being self-renewing and multipotent, having the ability to generate into diverse types of differentiated cells. As such, they show promise in the treatment of neurological disorders or trauma. The synaptic connections involved in neural circuits are continuously altered throughout the life of the individual. However, due to synaptic plasticity and cell death, neurogenesis (the generation of new neurons) was thought to be complete early in the postnatal period. The discovery of multipotent neural stem cells (MNSCs) in the adult brain (see, e.g., Alvarez-Buylla et al., 1997. Neurobiol. 33: 585-601; Gould et al., 1999, Science 286: 548-552) has significantly changed the theory on neurogenesis, given that the presence of MNSCs in the adult brain suggests that regeneration of neurons can occur throughout life.
Over the last few years it has become clear that neurons are generated continuously from adult stem cells in “germinal zones” of the adult brain in rodents (McKay, R., 1997, Science 276, 66-71; Gage, F. H., 2000, Science 287 (5457), 1433-1439) The adult brain retains a reservoir of neural progenitors in the dentate gyrus (DG) of the hippocampus and the rostral sub-ventricular zone (SVZ) that can proliferate in response to ischemic injury, trauma (Arvidsson, A. et al., 2002, Nat Med 8 (9), 963-970; Kernie, S. G. et al., 2001, J Neurosci Res 66 (3), 317-326), hypoxia, epilepsy, focal injury (Parent J M. et al., 2006, Ann Neurology 59(1), 81-91; Ernst C, Christie B R., 2006, Journal of Neuroscience Research 83(3), 349-361; Suh S W. et al., 2005, Diabetes 54(2), 500-509) and to other stimuli such as exercise or provision of an enriched environment (Kempermann, G. et al., 1997, Nature 386 (6624), 493-495; van Praag, H. et al., 1999, Nat Neurosci 2 (3), 266-270; Kempermann, G. et al., 2002, Ann Neurol 52 (2), 135-143). The entire ventricular wall of the human brain has been shown to be lined with a “ribbon” of neural progenitors capable of generating neurons in vitro, but the fate of these progenitors in vivo appears limited to glial lineages (Sanai, N. et al., 2004, Nature 427 (6976), 740-744). Other regions of the brain such as the cerebral cortex and the midbrain have been reported to have potential for neurogenesis, though these findings remain controversial (Gould, E. et al., 1999, Science 286 (5439), 548-552; Magavi, S. S. et al., 2000, Nature 405 (6789), 951-955; Zhao, M. et al., 2003, PNAS 100 (13), 7925-7930; Frielingsdorf, H. et al., 2004, PNAS 101 (27), 10177-10182).
Bone marrow is well known to house stem cells that regenerate all the blood lineages throughout life. A growing focus of research has been on the discovery of more “primitive” multipotent stem cells in bone marrow that are capable of giving rise to tissues of all embryonic germ layers (Jiang, Y. et al., 2002, Nature 418 (6893), 41-49). Bone marrow contains at least two types of stem cells, hematopoietic stem cells and stem cells of non-hematopoietic tissues variously referred to as mesenchymal stem cells or marrow stromal cells (MSCs) or bone marrow stromal cells (BMSCs). BMSCs are easily isolated from a small aspirate of bone marrow and they readily generate single-cell derived colonies. The single-cell derived colonies can be expanded through as many as 50 population doublings in about 10 weeks, and can differentiate into osteoblasts, adipocytes, chondrocytes (Friedenstein et al., 1970, Cell Tissue Kinet. 3: 393-403; Castro-Malaspina et al., 1980, Blood 56: 289-301; Beresford et al., 1992, J. Cell Sci. 102: 341-351; Prockop, 1997, Science 276: 71-74), myocytes (Wakitani et al., 1995, Muscle Nerve 18: 1417-1426), astrocytes, oligodendrocytes, and neurons (Azizi et al., 1998, Proc. Natl. Acad. Sci. USA 95: 3908-3913); Kopen et al., 1999, Proc. Natl. Acad. Sci. USA 96: 10711-10716; Chopp et al., 2000, Neuroreport II 3001-3005; Woodbury et al., 2000, Neuroscience Res. 61: 364-370).
Adult bone marrow retains a reservoir of multi-potent stem cells and BMSCs can be used as an alternative source of multi-potent stem cells. Bone marrow-derived cells have been shown by independent investigators to give rise to neural cells and these may migrate to brain where they appear to differentiate into neurons and glia (Sanchez-Ramos, J. et al., 2000, Experimental Neurology 164, 247-256; Woodbury, D. et al., 2000, Journal of Neuroscience Research 61 (4), 364-370; Mezey, E. et al., 2000, Science 290 (5497), 1779-1782; Brazelton, T. R. et al., 2000, Science 290 (5497), 1775-1779). The mechanism for transdifferentiation of bone marrow to neural cells is not clear and may reflect the capacity of bone marrow derived cells to fuse with injured neurons (Weimann, J. M. et al., 2003, Proc Natl Acad Sci USA 100 (4), 2088-2093). Fusion mechanism aside, there are many experiments in vitro that demonstrate subpopulations of bone marrow cells (and single derived clones) can be induced to differentiate into neural phenotypes (Sanchez-Ramos, J et al., 2000, Experimental Neurology, 164:247-256; Woodbury D, et al., 2000, Journal of Neuroscience Research 61 (4), 364-370; Kohyama J, et al., 2001, Differentiation 68:235-244; Jiang Y, et al., 2002, Nature 418:41-49; Jin K, et al., 2003, Experimental Neurology 184:78-89).
U.S. Patent Application 2005/0169896A1 to Li et al. provides further evidence to establish that BMSCs can differentiate into neurons and these cells can have added therapeutic benefit in CNS injuries and diseases when transplanted into a patient suffering from a neurodegenerative disease or neural injury. It was found that transplanted bone marrow cells activated the endogenous stem cells and ependymal cells in the brain to proliferate and differentiate into parenchymal cells including neurons. These bone marrow cells were found to have produced an array of factors, including cytokines and growth factors that promote repair and plasticity of the brain.
Additional evidence of BMSCs differentiating into neurons was substantiated by this laboratory. We found that both nestin-enriched cells derived from bone marrow and adult brain neural stem cells can differentiate into cells with the morphological, immunocytochemical, and functional characteristics of neurons (Song S, et al., 2007, Stem Cells and Development 16:747-756).
It has been well established that both brain and bone marrow retain an endogenous population of stem cells that proliferate in response to environmental and pharmacological manipulations. These stem cells can replace those cells lost in some experimental lesions. However, neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's diseases are characterized by continuous loss of neurons. The prevalence of these human neurodegenerative diseases underscores the inadequacy of self-repair through neurogenesis.
Many neurological deficits are localized to particular brain regions. A “neurological deficit” is a condition caused by a wide variety of diseases and injuries, including age, trauma, malfunction, and degeneration, that causes loss of brain and/or nervous system function. Neuronal degeneration in specific brain locations can lead to the inability of the brain to synthesize and release neurotransmitters that are necessary to neural signaling.
Alzheimer's disease is due to a degenerative process that is characterized by the progressive loss of cells from the basal forebrain, cerebral cortex, hippocampus, and other brain areas. Acetylcholine-transmitting neurons and their target nerves are particularly affected. Alzheimer's disease results in impaired memory, thinking, and behavior. Alzheimer's disease is characterized by a buildup of amyloid beta protein, which produces neuritic plaques between the neurons, and tau protein neurofibrillary tangles in which the protein strands twist around each other and damage local neurons.
Parkinson's disease is another progressive neurodegenerative disease. The brain areas with the most regularly observed changes have been in the aggregates of melanin-containing nerve cells in the brainstem (substantia nigra, locus coeruleus). Parkinson's disease affects the central nervous system and is often characterized by tremors, stiffness of joints, difficulty in movement, and speech difficulties.
Huntington's disease is another neurodegenerative disease which affects the central nervous system and is characterized by unsteady gait, abnormal posturing, rigidity, and impaired psychomotor functions. Huntington's disease affects the striatum and the substantia nigra, as well as other areas of the brain including the cerebral cortex, cerebellum, and hippocampus.
The inability of neurogenesis by endogenous neural stem cells or neuronal rescue by bone marrow derived cells to keep pace with neurodegeneration has underscored the need for a method to induce endogenous stem cells to proliferate, migrate, and differentiate in order to promote the self-repair mechanisms of the brain. We have discovered that micro-stimulation of a brain region by transient insertion and removal of a fine needle will stimulate the self-repair mechanisms of the brain.